Spatial and stratigraphic distribution of sedimentological and diagenetic features of an Aptian carbonate reservoir of the Santos Basin, Brazil.

The Aptian carbonate rocks of Santos Basin, in the Brazilian southeastern continental margin, show reservoir quality strongly controlled by the characteristics of the depositional environment and associated diagenetic alterations. This study presents a quantitative approach based on petrography, mineralogy, and geochemical data of the Barra Velha Formation to integrate spatial and stratigraphic distribution of sedimentological and diagenetic aspects and provide insights into the parameters controlling the genesis and distribution of the diagenetic phases of Pre-Salt carbonate reservoirs of Santos Basin.

The Barra Velha Formation consists of a heterogeneous mix of in-situ and reworked grains. The in-situ facies are composed of fascicular calcite (shrubs), spherulites, microcrystalline calcite, and clay minerals, whereas the reworked facies present different proportions of shrubs and spherulite fragments, siliciclastic/volcanoclastic grains, and variable clay content. The facies with higher Mg-clay content predominate in lower structural positions and regions with low relative relief. Higher proportions of spherulites occur in the transitional zones, and fascicular calcite occurs preferentially in the transitional and higher structural areas. Grainstone facies are observed in all depositional environments but with variations in their composition (intraclasts, siliciclastic/volcanoclastic grain, and clay content). Stratigraphically, facies with higher Mg-clay content predominate in Units 3 (base) and 2 (intermediate), whereas Unit 1 (top) is marked by a higher proportion of grainstones and fascicular calcites. In Unit 1, fascicular calcite crusts tend to expand from the structural high toward lower areas.

The main diagenetic products and their association with different facies highlight the roles of depositional settings and primary constituents in the diagenetic processes. The tectonic and climatic context of the BVF favored Mg-clay precipitation that accumulated preferentially in lower structural portions of the basin. Mg-clays also show a preservation pattern strongly linked to the structural setting, with higher structural portions and fault zones more susceptible to their major diagenetic alterations. Dolomite is a major diagenetic phase in the studied samples, followed by silica. These diagenetic phases are largely associated with Mg-clay alteration and predominate in Unit 1. The dissolution of Mg-clays and high alteration of the carbonate phases are concentrated in higher structural positions. Given the higher occurrence of faults in these areas and the presence of saddle dolomite, barite, celestine, and fluorite, which can be associated with hydrothermal alterations, part of the dissolution processes may be related to hydrothermal processes. The depositional and diagenetic aspects indicate a close connection between the hydrochemical evolution of the lake waters with the origin and diagenesis of these deposits.